A typical paint spray gun comprises a gun body with an integral handle pendant from one end of the body and a spray head provided at the opposite end of the body. At an extremity of the integral handle is an air inlet to which a supply of compressed air can be connected. The air inlet extends into a passage which passes though the handle of the gun to the gun body. A paint inlet is provided in an outer surface of the body of the gun. The paint inlet extends into a paint passage also passing through the gun body. The incoming air encounters the incoming paint and atomises it at the spray head thereby producing the paint spray.
The flow of both air and paint through the gun body is controlled by valves. Typically, both valves can be opened by means of a single trigger mounted on the handle.
In practice, it is often desirable to open only the air valve in the first instance so that a work piece may be “blown down” prior to painting. This involves blasting of the work piece with pressurised air so as to remove dust from the surface prior to painting. This is typically achieved by the operator applying light pressure to the trigger in the first instance which causes the air valve to open, and subsequently to apply greater pressure, which maintains the air valve open and opens the paint valve.
The type of valve typically used to control air flow is known as a poppet valve. Examples of prior art poppet valves are illustrated in the accompanying figures and are described in more detail below. When these valves are closed, an excess of air pressure acting on the rear of the valve compared to in front of the valve applies a load which needs to be overcome when the valve is first opened. Once the valve is partially open, air pressure around the valve begins to equalise and relatively less load need be applied to further open the valve. In applying the higher load so as to open the air valve, it is known for operators to inadvertently overshoot the position where only the air valve is open, opening the paint and air valves and sputtering the work piece with paint rather than blow it down.
Previous attempts to reduce the initial load required to open the valve have focussed on reducing the area of the air valve. However, as air valve area decreases, pressure drop across the valve increases. Excessively high pressure drops are unacceptable as these require high pressures at the air inlet, which can prove difficult to supply in many workshops.